1. Field of the Invention
This invention relates to seismic geophysical surveying. More particularly it relates to a type of explosive charge known as a linear charge, a constant detonation-velocity charge, or a velocity-matching charge, in which the average velocity of detonation of the charge along its length, is substantially the same over equal increments of length of the charge.
Still more particularly it relates to a multi-unit, serially-connected charge, having improved means for detonating from one unit to the next, and including the provision of synchronously detonating booster explosive units, as the detonation wave passes the joints between each of the units.
2. Description of the Prior Art
In seismic geophysical surveying, wherein artificial seismic waves are created by detonating an explosive charge in the earth, and the resulting elastic or seismic waves are received by geophones, or other sensors, variously disposed on the earth's surface, it has become generally recognized that a long continuous explosive charge, or an array of many small point charges properly spaced and timed, such as to match the timing to the advancing of a seismic wave front through the adjacent earth medium, offers substantial advantages. Such elongated charges, or arrays, have directional properties, in that the created seismic waves travel in a preferred direction with maximum strength, and at the same time cause less unwanted disturbances travelling in other than the preferred direction.
In the prior art an example of the constant detonation velocity charge is described in U.S. Pat. No. 3,150,590 of Daniel Silverman. Use of this type of charge has been made in the geophysical industry where its particular advantages of directivity of energy delivery, and higher frequency content of the seismic energy, have proved useful in the solution of particular seismic problems.
However, a number of deficiencies and disadvantages of this particular design have been found, including high cost, and high weight. A less expensive version of this design utilized a paper tube, in place of the wood stick. But this had other more serious disadvantages. In particular, the charges lacked capability of submersion in water in the shotholes for long periods. More serious, however, was the occurrence of a great number of misfires; that is, failure to detonate from one explosive unit to the next.
There was a third serious disadvantage, that involved the use of concentrated explosive charges in the form of cans of explosive, that could be screwed in position at the couplings between the serial constant-velocity units.
In the use of constant velocity charges, there is a limit to the total weight of explosive that can be used on a charge of a given length. Although a given weight of constant velocity charge, because of its directivity, is equivalent, for seismic recording purposes, to several times that weight of concentrated point charge, sometimes additional explosive weight is needed. This was provided by inserting cans of concentrated explosive in some of the joints between units.
However the commercial cans of explosive were only available in substantial weight units. If only a small increment of charge weight was needed, the addition of 1 or 2 cans placed in the charge would spoil the directivity effect of the entire charge, and the principal advantage of the constant velocity charge would be lost.